The separation of aromatics from non-aromatics is useful in upgrading aromatics containing streams in petroleum refineries, such streams including, naphtha streams, heavy catalytic naphtha streams, intermediate catalytic naphtha streams, light aromatic streams and reformate streams, and in chemical operations for the recovery of aromatics such as benzene, toluene, xylenes, naphthalene, etc.
The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community. Methods of membrane separation include hyperfiltration (also known as reverse osmosis in aqueous separations), pervaporation and perstraction. Pervaporation relies on vacuum on the permeate side to evaporate the permeate from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. In perstraction, the permeate molecules in the feed diffuse into the membrane film, migrate through the film and reemerge on the permeate side under the influence of a concentration gradient. A sweep flow of liquid or gas is used on the permeate side of the membrane to maintain the concentration gradient driving force. In contrast, hyperfiltration does not require the use of external forces on the permeate side of the membrane, but drives the separation through application of a pressure gradient.
Membrane separation of aromatics from saturates has been the subject of numerous patents.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatic, unsaturated, saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons e.g., aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
U.S. Pat. No. 4,929,358 teaches the use of polyurethane membranes for the separation of aromatics from non-aromatics. Permeation is conducted under pervaporation, perstraction, reverse osmosis, or dialysis conditions. None of the experimental results reported in this patent were obtained under reverse osmosis conditions.
Polyimide membranes have been used for the separation of aromatics. U.S. Pat. No. 4,571,444 teaches the separation of alkylaromatics from aromatic solvents using a polyimide polymer membrane. The polyimide membrane of choice was an asymmetric polyimide polymer membrane prepared from a fully imidized, highly aromatic polyimide copolymer. Permeation was performed under reverse osmosis conditions.
U.S. Pat. No. 4,532,029 discloses the use of an asymmetric polyimide membrane for the separation of aromatics from lower aromatic middle distillate feeds. Permeation of the feeds in the presence of a light polar solvent, e.g., acetonitrile, was required to obtain permeates having a high aromatic content, i.e., greater than 86%.
The majority of investigations for aromatic/non-aromatic separations have heretoafore involved pervaporation or perstraction separation techniques. This is probably due to reports of prior literature that very high operational pressures are required in hyperfiltration to reach a equivalent performance achievable by pervaporation and perstraction processes. Unfortunately, pervaporation and perstraction separation systems are higher cost than a hyperfiltration system due to expenses associated with vacuum, refrigeration and heat transfer systems.
Consequently, it is an advantage of this invention to provide improved asymmetric polyimide membranes for the separation of aromatic hydrocarbons from non-aromatic hydrocarbons in a feed stream by hyperfiltration. It is also an advantage of this invention to provide a method of preparing the membrane by a phase inversion technique, which method permits variations in processing conditions to optimize the selective permeation of aromatic hydrocarbons through the membranes in the presence of non-aromatic hydrocarbons.
Another advantage of the invention is to provide a membrane useful in a process of separating aromatic hydrocarbons as described in copending application Ser. No. 125,256, entitled "Recovery of Aromatic Hydrocarbons Using Lubricating Oil1-Conditioned Membranes", Mobil filed on even date herewith.
Other facets and advantages of the present invention will be apparent from the ensuing description and the appended claims.